DESCRIPTION (the applicant's description verbatim): Sudden Cardiac Death is a major cause of mortality in the United States. In an effort to attack this problem, the concept of a Chain of Survival has been presented as a way to characterize the tasks necessary for a successful resuscitation. Early defibrillation is central to improved survival rates following out of hospital cardiac arrest. Very little is known about defibrillation efficacy of spontaneous arrhythmias in the presence of acute ischemia. A much larger shock is needed to halt ventricular fibrillation that occurs spontaneously in the presence of acute ischemia than to halt electrically induced ventricular fibrillation in the non-ischemic heart. We will study the activation patterns following successful and failed defibrillation shocks with the heart in four conditions designed to model different aspects of ischemic ventricular fibrillation: (1) acute regional ischemia causing spontaneous ventricular fibrillation, (2) acute regional ischemia followed by electrically induced ventricular fibrillation, (3) acute regional ischemia causing spontaneous ventricular fibrillation which in turn causes prolonged global ischemia, and (4) acute regional ischemia causing spontaneous ventricular fibrillation in the setting of an old myocardial infarction. We will test the hypothesis that, in the setting of an ischemically induced spontaneous arrhythmia, a defibrillation shock must do three things: (1) stop all fibrillation wavefronts, (2) not restart fibrillation, and (3) stop the trigger of the original arrhythmia. We will also test the hypothesis that a major determinant of whether or not a spontaneous arrhythmia caused by acute ischemia can be easily defibrillated depends upon the mechanism of initiation of the arrhythmia and duration of the arrhythmia. To test these hypotheses, we will use electrical mapping techniques to map the initiation of ventricular arrhythmias, and then map the first few post-shock activations following delivery of a defibrillation shock. By developing an understanding of how a defibrillation shock succeed or fails under these conditions, we will be in a better position to develop new defibrillation techniques that will hopefully increase survival rates of patients suffering an episode of sudden cardiac death.